1. Field of the Invention
The present invention relates to various approaches for processing an ongoing stream of digital samples which, when employed together, are suitable for use in a vestigial-sideband (VSB) digital modulator that derives a 6 MHz bandwidth input signal selectively centered either at 63 MHz (Channel 3), 69 MHz (Channel 4) or 5.38 MHz IF (baseband) for a television receiver and, more particularly, for a high-definition television (HDTV) receiver. Related applications filed concurrently herewith are COMPLEX MODULATOR CARRIER SIGNAL GENERATOR Sn. (09/382,234); A PULSE CODE MODULATED TO DC CENTERED VSB CONVERTER 09/382,232, U.S. Pat. No. 6,229,464; and VSB DIGITAL MODULATOR 09/382,231.
D/A Converters input a discrete sequence of digital sample values and output analog values. The analog value corresponding to a single input digital value is maintained for the interval between input sample values. In the sample data digital domain the impulse train of discrete values in time has a periodic frequency spectrum. The D/A "hold" operation modifies the periodic spectrum by multiplication with a sin(.pi..multidot.f/fs)/(.pi..multidot.f/fs) {termed sin(x)/x}, where f is the analog frequency in Hz and fs is the digital sample rate in samples per second. The sin(x)/x frequency response is not periodic and is defined on the interval .omega..epsilon.(-.infin..infin.).
In conventional applications the sin(x)/x frequency characteristic of the D/A is compensated with a fixed coefficient pre-filter with x/sin(x) frequency response in the interval f.epsilon.(-fs/2, fs/2). This pre-filter is often packaged with the D/A. Note that frequencies outside this interval are not compensated correctly.
The present invention addresses sinx/x correction in the context of Direct Digital Synthesis of RF modulated signals (esp. 8/16 VSB modulated carriers of the US HDTV Standard) wherein:
1. The desired RF image may or may not be in the first nyquist region, (-fs/2, fs/2) of the D/A converter, and the desired image contains a selected TV channel band for transmission. PA0 2. The lowest sample rate of interest in the system is the symbol rate of the digital modulation, which determines the information bandwidth. PA0 3. The highest sample rate in the system is the D/A output rate. This will be typically at Ntimes the symbol rate. PA0 4. It is only necessary to compensate the band to be transmitted and therefore sin(x)/x compensation may be performed as early in the transmission chain as the band is resolvable. For VSB modulation this can be done at the symbol rate, and hence before, or after sample rate conversion, and/or before or after up modulation. PA0 5. Correction can be performed with real or complex filtering depending on its carrier frequency at the point of correction.
For the HDTV signal, correction is performed at a point in the modulator where the data is complex with carrier frequency of 0, which necessitates complex filtering.
2. Description of the Prior Art
Reference is further made to U.S. Pat. No. 5,208,596, entitled "DAC Distortion Compensation", which issued May 4, 1993 to Charles B. Dietrich and is assigned to the same assignee as the present application. The teaching of this patent is representative of a prior-art approach to compensating for the inherent sin x/x roll-off in the analog output magnitude of a digital-to-analog (D/A) converter as a function of frequency.